1. Field of the Invention
The invention relates to a bearing arrangement and a method of preparing a bearing arrangement for a roll journal of a roll for rolling metal rolled stock such as, e.g., slabs or metal strips. The bearing arrangement includes a bearing insert with a bearing bush and a journal bush which is located in the bearing bush with a bearing clearance and is pushable over the roll journal. The invention relates in particular to bearing arrangements for working and back-up rolls in rolls mills of the metallurgical industry.
2. Description of the Prior Art
Such bearing arrangements and methods of their preparation are basically known in the state-of-the art, see, e.g., R. S. Schrama in “Iron and Steel Technologies,” December 2005, “Types of Failures of Back-up Roll Assemblies.”
A known bearing arrangement and a known method of its preparation will be described in detail with reference to FIGS. 1-4.
FIG. 1 shows a known bearing arrangement 100 for supporting a roll 200. The roll includes a roll body 220 and two roll journal 210. Each of the roll journals 210 is supported by a separate bearing arrangement 100. Each of the bearing arrangements includes a bearing housing also called a bearing insert.
FIG. 2 shows a detailed construction of the bearing arrangement. Thus, the bearing arrangement 110 includes an annular bearing bush 112 for receiving the roll journal 210 and which is arranged in the insert. The bearing arrangement 100 includes, in addition to the bearing bush, an annular journal bush 120 that is pushed on the roll journal 210. In order to prevent rotation of the roll journal 210 in the journal bush 120, there are provided two keys 142, 144 between the journal bush 120 and the roll journal 210. A pressure shoulder ring 130 prevents a sidewise displacement of journal bush from the journal during the rolling operation.
The journal bush 120 has a radial elevation in form of a band 122 at its end remote from the roll body. This annular band has a certain axial extension for taking up forces which are generated, e.g., during pulling of the journal bush 120 from the roll journal. To this end, the insert, together with the bearing bush 112, is pulled in the axial direction away form the roll body 220, with the insert or the bearing bush 112 engaging the band and, in this way, pulling the journal bush in the axial direction from the roll journal, in the axial direction meaning in the direction of the longitudinal axis of the roll 200.
In FIG. 2, the force distribution acting on the bearing arrangement during the rolling operation is shown with arrows.
FIG. 3 shows a detailed view of the bearing arrangement in an initial or new condition. FIG. 3 shows a bearing clearance between the outer diameter Da of the journal bush and the inner diameter Di of the bearing bush. In a new condition, this bearing clearance lies, dependent on the type of the bearing arrangement, in an allowable value region of 0.5 to 1.5% of an initial outer diameter of the journal bush.
During rough rolling operation, the bearing arrangements are subjected to strong mechanical and thermal loads, and, as result, wear off. The wear phenomenon is shown in form of drag lines on or damages of the surface of the inner side of the bearing bush and an outer and inner side of the journal bush. The repair of such bearing arrangements is a conventional practice. Traditionally, both the journal bush and the bearing bush are repaired. Traditionally, the repair of the roll journal 120 includes the following steps: machining the outer side of the journal bush 120 by turning, milling, or grinding, whereby the outer diameter of the journal bush is reduced to such an extent that the drag lines or damages are no more visible. In the same way, the inner side of the journal bush can be treated, which increases the inner diameter. However, the drawback of the increase of the inner diameter of the journal bush consists in that at a later re-use of the journal bush, it is pushed on the roll journal further than originally. The drawback of this consists in that a seal between the roll body and the insert, not shown in FIG. 4, would be compressed in an unacceptable degree. Therefore, it becomes necessary to shorten the journal bush at its end adjacent to the roll body by an amount C, so that the journal bush 120 could not be displaced on the roll journal further than in a new condition.
The drawback of reduction of inner diameter of the journal bush 120 at its end remote from the roll neck consists in that the band 122 of the journal bush 120 rises too close to the bearing bush. A certain minimum distance A should be retained between the band 122 and the bearing bush 112. In order to retain this distance A, with an increase of the inner diameter of the journal bush, the band 122 should be reworked, i.e., to be so reduced in its axial extension that the minimum distance A is reinstated. However, the axial extension of the band 122 should not fall below a minimum amount to insure the necessary stability of the band against the above-mentioned sidewise displacement of the journal bush from the roll journal.
A further drawback of the increase of the inner diameter of the journal bush consists in that the end side of the journal bush remote from the roll body becomes spaced by a certain amount from the press shoulder ring 130 when the journal bush is pushed over the roll journal and, as a consequence, the pressure shoulder ring should correspondingly also be pushed further.
Manufacturing of the bearing bush with a coating of a bearing metal, preferably, while metal is discloses in the prior art, e.g., in EP 1 151 145 B1.
The repair of the bearing bush includes the following steps:
Removing a layer of bearing metal adjacent to the journal bush, e.g., white metal layer, in order to remove the drag lines and damages. The white metal layer is usually comparatively thin in the initial condition of the bearing bush so that it can be removed only completely. As a rule, during the removal of the white metal layer, disadvantageously, a thin layer of a carrier material remote from the journal bush, usually steel of which the bearing bush primarily consists, is also removed with the white metal layer, whereby the bearing bush is weakened with each repair to a certain degree. After the removal of the white metal layer, a new layer of white metal is put on the inner surface of the bearing bush 112 on the old/previous predetermined nominal diameter Di old, this time with a greater thickness of the white metal layer than the thickness of the removed old material layer. The greater thickness layer of the new white metal compensates the removed carrier metal. Though, as discussed, the inner diameter Di old of the bearing bush remains unchanged after putting of the new bearing metal layer for purposes of interchangeability, in particular, does not become smaller, the outer diameter Da of the journal bush is reduced as a result of removal, so that the bearing clearance increases with each removal of outer side of the journal bush. The bearing clearance between the white metal layer and the outer side of the journal bush should not be greater than 1.7% of the original outer diameter of the journal bush after treatment. As soon as the maximal allowable value of 1.7% for the bearing clearance is reached or the inner diameter of the journal bush exceeds a predetermined threshold, the journal bush should not be further repaired but should be replaced.
Traditionally, the journal bush is replaced after two-three repairs, which is usually associated with high costs for the user and a large consumption of resources, while the journal bush usually must be scrapped.
The thickness of the bearing metal layer, in particular, white metal layer, in the bearing bush has a thickness threshold SD that should not be exceeded. This is because the white metal is substantially weaker than, e.g., carrier material of the bearing bush on which the white metal is put. The white metal layer can be only increased so far that the load requirement to the bearing arrangement can be met with, simultaneously, retaining the predetermined nominal diameter.
For this reason, such repair of a bearing bush is not usually often, but is rather made at most two-three times.